The term “active layer” refers to the relatively thin
layer of ground between the surface and permafrost that undergoes seasonal
freezing and thawing (Muller, 1947; Burn, 1998). Across this layer energy and
water are exchanged between the atmosphere and underlying permafrost. Because
most biological, physical, chemical, and pedogenic processes take place in the
active layer, its dynamics are of interest in a wide variety of scientific and
engineering problems.

The
definition given above is based entirely on thermal criteria, without regard to
material composition or properties. The volume and properties of the active
layer are highly variable in time and over space. Idealized diagrams showing
smooth trends of active-layer thickness (ALT) across latitudinal gradients
(e.g., Brown, 1970, Figure 4, p. 8) can be misleading because variations in
vegetation, substrate properties, and water content can result in very large
differences in ALT, even over small distances (Nelson et
al., 1999; Hinkel and Nelson, 2003). Temporal changes, particularly surface
temperature and moisture conditions, can also lead to substantial year-to-year
differences in ALT, even at fixed locations. For these reasons, it is necessary
to monitor ALT using well-defined measurement and sampling techniques. The
Circumpolar Active Layer Monitoring (CALM) program was developed to provide
standards of measurement and a comprehensive database describing the history and
geography of ALT and other selected parameters at a large number of sites
representative of permafrost terrain (Brown et
al., 2000).

This
note discusses several common methods used to measure the thickness of the
active layer, including mechanical probing, frost/thaw tubes, soil temperature
profiles, and remote methods such as ground penetrating radar (GPR) and
satellite measurements. Only nondestructive methods are discussed. Many of the
methods and sampling designs described in this note were developed or refined by
various investigators working in association with the CALM program.

The
term “active-layer thickness” is used in reference to the maximum
development of the thawed layer, reached at the end of the warm season (van
Everdingen, 1998). This is distinct from the term “thaw depth,” used here to
refer to the thickness of the thawed layer at any time during its development in
summer. Stated alternatively, thaw depth is an essentially instantaneous value
that is always less than or equal to the thickness of the fully developed active
layer. Active-layer thickness can vary substantially on an interannual basis. In
general, it is greater in years with warmer summers and thinner in those with
cooler temperatures (Brown et al., 2000). Both thaw depth and active-layer thickness display
large spatial variability over short lateral distances and, because rates of
thaw vary in response to the properties of surface and subsurface materials, the
spatial variability of thaw depth increases over the course of a single summer
(Nelson et al., 1997). The magnitude
of active-layer variability can be quantified and used to characterize various
types of permafrost terrain (Nelson et al., 1998; Hinkel and Nelson, 2003).

Observation methods

Probing

Probing
of the active layer is performed mechanically with a graduated rod. The typical
probe is a 1 m long stainless-steel rod with a tapered point, is 1 cm in
diameter, and has an attached handle (Figure 1). At sites where thaw depth is
very large (e.g., 1-3 m), the diameter of the probe must be also be greater to
withstand the bending stress generated by insertion. It is very difficult,
however, to extract a probe in deeply thawed soils, and this problem is
exacerbated if the probe’s surface area is very large.

Figure
1: Steel 1-m probe, graduated in 1-cm increments. This model breaks down
into three pieces for transport in handle, and has additional flights. This
design was developed at the U.S. Army Cold Regions Research and Engineering
Laboratory in the 1970s.

The
probe rod is inserted into the ground to the point of resistance. A gentle
pumping motion is used to gradually force the rod progressively deeper into the
thawed ground without bending. A distinctive sound and feel is apparent when
ice-rich frozen ground is encountered. The rod is grasped with the hand, and the
hand is slowly slid down the rod to the top of the soil material; i.e., to the
base of the vegetation. The rod is grasped firmly and removed, using the fingers
to carefully mark the position on the rod. Thaw depth is then read from the
graduations and recorded. All measurements are made relative to the surface; in
standing water, both thaw depth and water depth are recorded.

Typically,
two measurements are made at each location and the average reported. If a
standard spacing is maintained between the two samples, the researcher has one
metric of thaw variability. Identical probing methods can be used to measure the
depth of the snow pack.

Advantages:Probes are relatively inexpensive and several models and sizes are
available (see vendors at http://k2.gissa.uc.edu/~kenhinke/CALM/equipment.html).
Measurements are relatively easy to make, and require little time. The primary
advantages of probing are: (a) its suitability for collecting large numbers of
measurements; (b) its ability to generate samples of data that are statistically
representative of local areas; and (c) it can be used in conjunction with
vegetation and soil information to estimate the volume of thawed soil over
extensive regions (e.g., Nelson et al.,
1997; Klene et al., 2001; Shiklomanov
and Nelson, 2002).

Although
single point measurements have utility, measurements along transects or on grids
enable the researcher to plot or map annual thaw patterns, make volumetric
estimates, and identify the factors responsible for active-layervariability (Nelson et al.,
1999; Gomersall and Hinkel, 2001). Because measurements can be made rapidly and
with little effort, probing is well suited for implementing formal sampling
designs (Nelson et al., 1998, 1999;
Gomersall and Hinkel, 2001). The length of transects or the dimensions of grids
depend on scientific objectives and the scale of the active layer’s local
variability, and a program of exploratory sampling may be necessary to establish
a grid of dimensions appropriate to the locale and the scientific goals (Nelson et
al., 1998, 1999). Grids are typically 10, 100 and 1000 m on a side, with
grid nodes spaced evenly at 1, 10, or 100 m, respectively (Brown et
al., 2000). Node locations are identified using stakes or other
semi-permanent markers, and replicate measurements are made at approximately the
same location each year to facilitate interannual comparison.

Disadvantages:Timing is one of the primary limitations with probing. Ideally, measurements
should be collected at the time of maximum thaw depth. Field measurements are,
however, often constrained by logistical or weather-related considerations.
Experience can help the researcher decide when to collect end-of-season
measurements, but the date usually varies from year toyear at each site. It is therefore unlikely that measurement of thaw
depth coincides perfectly with the actual active-layer thickness. However,
because thaw progression is usually proportional to the square root of the time
elapsed since snowmelt, late-season thaw-depth measurements generally correspond
closely with the maximum thickness of the active layer.

More
intractable problems with probing arise when substrate properties prevent
accurate determination of the frost table’s position. In some cases, the top
of the frozen (ice-bearing) zone does not coincide with the position of the
frost table as defined by the 0°C
criterion. The relation is dependent on soil salinity, particle size, and
temperature. Well-drained sands and gravels may contain too little interstitial
ice for adequate resistance to probing to develop. In saline or extremely
fine-grained soils probing can yield inaccurate estimates owing to the presence
of unfrozen water. Under such conditions it may be possible to calibrate
mechanical probing using a thermal probe (Mackay, 1977; Brown et
al., 2000, p. 172). Readings may be very difficult to obtain in stony
substrates such as glacial till (see paper by S. Hanson, this volume). Probing
cannot ascertain if thaw subsidence has occurred.

Frost/thaw tubes

Thaw/frost tubes are devices
extending from above the ground surface through the active layer into the
underlying the permafrost. They are used extensively in Canada. Construction
materials, design specifications, and installation instructions are available
for several variants of the basic principle (Rickard and Brown, 1972; Mackay,
1973; Nixon, 2000). A schematic for a recent
design is shown in Figure 2.

Figure 2. Schematic diagram
of a recent frost/thaw tube design implemented by the Geological Survey of
Canada. This design incorporates a scribing device to record thaw settlement
induced by ablation of an ice-rich layer near the interface between the active
layer and permafrost table. Diagram courtesy of F. M. Nixon, Geological Survey
of Canada. From Nixon et al. (1995).

A rigid outer tube is anchored in
permafrost, and serves as a vertically stable reference; an inner, flexible tube
is filled with water or sand containing dye. The approximate position of the
thawed active layer is indicated by the presence of ice in the tube, or by the
boundary of the colorless sand. Each summer the thaw depth, surface level, and
maximum heave or subsidence is measured relative to the immobile outer tube.
These measurements are used to derive two values for the preceding
summer: (1) the maximum thaw penetration, independent of the ground surface and
corrected to a standard height above the ground established during installation;
and (2) the active-layer thickness, assumed to coincide with maximum surface
subsidence. With modifications, the accuracy of the measurements is about 2 cm.

Advantages:The primary advantage of frost/thaw
tubes is that they provide
an inexpensive annual record of both maximum thaw penetration and active-layer
thickness, although it is not possible to
determine the date. Because thaw tubes are durable, a multi-year
record is available for comparison. Thaw tubes
are especially useful in areas where thaw is too deep to monitor by probing, and
in stony, fine-textured, or saline substrates in which probing is not feasible.
Because the device is embedded in permafrost, the outer tube serves as a stable
reference to determine if thaw subsidence or heave has occurred (Nixon, 2000).

Disadvantages:Thaw tubes have several limitations. First, each tube records only a
single point measurement, which may or may not be representative of a larger
area. Second, installation typically entails drilling, and the difficulty and
time required may prohibit multiple tube installation at a site. Finally, there
may be significant disruption of the surface and subsurface materials during
installation, and the structure itself may influence local heat and moisture
flow. It is likely that the optimal situation is to install a thaw tube at
long-term sites, and collect additional measurements derived from spatially
extensive sampling such as probing. Over time, a comparative record can be
developed.

Soil temper

ature profiles

Soil and air temperature are
recorded as basic information at many CALMsites, especially with the increasing availability of inexpensive,
reliable temperature data loggers. Temperature sensors (usually thermistors) are
inserted into the active layer and upper permafrost as a vertical array. Several
CALM installations currently use an array of thermistors embedded in a
small-diameter acrylic cylinder (Figure 3) and connected to a high-capacity data
logger.

Figure 3. Thermal probe
incorporating one external and 12 internal thermistor sensors in fixed positions
within a rigid acrylic tube. The probe employs sophisticated electronics and is
usually connected to a large-capacity data logger capable of making and
recording high-frequency readings. The probe illustrated here was originally
designed by the U.S. Army’s Cold Regions Research and Engineering Laboratory,
and is manufactured by MRC (Measurement Research Corporation), Gig Harbor,
Washington USA.

Temperature records from a
vertical array ofsensors can be
used to determine active-layer thickness at a point location. The thickness of
the active layer is estimated using the warmest temperatures recorded at the
uppermost thermistor in the permafrost and the lowermost thermistor in the
active layer. The temperature records from the two sensors are interpolated to
estimate maximum thaw depth during any given year. For this reason, the probe
spacing, data collection interval, and interpolation method are crucial
parameters in assessing the accuracy and precision of the estimate.

Advantages:The advantages are similar to those for
frost tubes. However, since temperature monitoring is already being preformed,
there is no additional cost. Further, it is possible to estimate the date of
maximum thaw with a reasonable degree of accuracy. Depending on whether probes
are in a fixed or floating configuration, it may be possible to determine if
thaw subsidence has occurred at the location. Numerical methods can be used with
high-frequency thermal observations to estimate the thermal properties of the
substrate, such as effective thermal diffusivity (e.g., Hinkel, 1997). Thermal
records can also be used to identify the operation of non-conductive
heat-transfer processes in the active layer, and can be related to
meteorological events at the surface (Hinkel et al., 1997, 2001; Kane et
al., 2001).

Disadvantages:Limitations are similar to those of frost tubes; thermistor strings
effectively comprise only a single point measurement. They are relatively
expensive. They are also subject to surface and installation disruptions,
including vandalism and disturbance by animals. The accuracy of the active-layer
thickness estimate is fundamentally limited by the vertical spacing of the
probes and the data-collection interval. Many field investigators use a simple
linear interpolation to estimate maximum seasonal thaw depth, although an
exponential best-fit can be employed if there are several thermistors. Estimates
of error should be calculated and reported.

Remote sensing methods

Ground penetrating radar (GPR)
has been used with some success to map active-layer thickness along transects.
Because water effectively absorbs electromagnetic pulses, profiling is most
effective in winter, when the ground is frozen and covered with snow. The method
relies on the principle that the active layer contains less ice than the
permafrost immediately below, resulting in a reflection horizon at the
interface. With careful local calibration, usually accomplished through coring,
estimates of thaw depth along a continuous profile can be made. The accuracy of
the estimates is incompletely known, but appears to be within ± 15% in fine
grained soils. The expense, however, is often prohibitive. It is likely that GPR
methodologies will continue to develop. Further details on the use of GPR in
active-layer investigations are available in Doolittle et al. (1990) and Hinkel et
al. (2001). Another ground-based approach was developed by McMichael et
al. (1997), who used the Normalized Difference Vegetation Index (NDVI) to
exploit known relations between vegetation units and ALT across a toposequence
in northern Alaska.

Satellite imaging systems hold
promise for monitoring thaw depth across large areas. In particular, synthetic
aperture radar, carried at appropriate wavelengths, may have sufficient energy
to penetrate the often-saturated active layer and return a signal to the
satellite receiver (Kane et al.,
1996). Interpretive, convergence-of-evidence approaches have been used by Peddle
and Franklin (1993) and Leverington and Duguay (1996) with some success,
although the derived classes of ALT were very broad.

All aircraft- or satellite-based
systems necessitate collection of training data on the ground for calibration
and verification of the signal processing algorithms. The impetus for such a
system may come from unmanned missions to Mars.

Other considerations

The issue of thaw
subsidence, alluded to at several locations in the preceding text, is a
critical issue and forms the focus of several field-based experiments in the
CALM program. Thaw subsidence (thaw settlement) refers to downward displacement
of the ground surface occurring when ice-rich permafrost thaws. Melting massive
ground ice, typically veins and/or lenses, leaves a void; the weight of
overlying material, combined with drainage of meltwater, can result in
appreciable subsidence at the surface. This consolidation may not be apparent in
ALT records obtained exclusively by mechanical probing

Imagine, for example, that the
typical active-layer thickness is 30 cm at a site. (Figure 4) Below this is a 5
cm thick layer containing a 3 cm thick ice lens. During one especially warm
year, the thaw front penetrates to a depth of 35 cm, causing the ice lens to
melt. The measured thaw depth would be only 32 cm, since the 3 cm of ice will
have melted and the ground surface subsided by approximately this amount. Thus,
the measured active-layer thickness will actually underestimate total thaw
penetration. Conversely, the ground surface may heave upward on a seasonal or
permanent basis with the growth of segregation ice. This is particularly
problematic in medium-textured mineral soils.

Probing may not detect thaw
subsidence or heave. For this reason, some recent investigations have employed frost defended bench marks or other methods of tracking changes in
surface elevation (Nixon and Taylor, 1998; Little et al., 2003). This may entail installing rigid poles or bars
several decimeters into the upper permafrost to ensure that there is no vertical
movement. The top of the bar or rod serves as a stable vertical reference point
to monitor subsidence and heave. Rock outcrops can also be used for surveying
purpose. Simple and inexpensive vertical displacement gauges have been developed
to monitor seasonal heave and subsidence. These devices are anchored into the
permafrost using a thin, flexible steel cable, and can also be used to monitor
long term surface displacement. A description of one of gauge is available on
the CALM web site (http://k2.gissa.uc.edu/~kenhinke/CALM/active_layer.html).

To an increasing degree,
differential global positioning systems (DGPS) technology is being used to
collect vertical control points to monitor and map surface movement (Little et
al., 2003). Mobile DGPS units can achieve vertical resolution of less than 1
cm under the proper operating conditions. In many applications, this precision
may be sufficient for the user.

Conclusions

Many methods are currently being
used to measure thaw depth in the active layer above permafrost. Each method is
associated with distinct advantages and disadvantages. Each relies on a specific
tool; in the words of Craftsmanä
spokesperson Bob Vila, it is a question of “using the right tool for the
job.”The researcher must decide on the most useful and practical
method given the scientific objectives, duration of the study, and availability
of funds. It is likely, however, that a mixture of methods will be most suitable
for long-term sites.

Acknowledgements

This work was supported by the
National Science Foundation under grants OPP-9612647, 9896238 and 0095088 to FEN
and OPP-9529783, 9732051, and 0094769 to KMH. Any opinions, findings,
conclusions, or recommendations expressed in this material are those of the
authors and do not necessarily reflect the views of the National Science
Foundation. Identification of specific products and manufacturers in the text
does not imply endorsement by the National Science Foundation.

Nixon
FM, 2000. Thaw-depth monitoring. In The Physical Environment of the Mackenzie
Valley, Northwest Territories: a Base Line for the Assessment of Environmental
Change. Geological Survey of Canada
Bulletin547: 119-126.